192 



nally even give the impression of occurring suddenly. Indeed, as 

 far as experience .goes, the change of apparently symmetrical sub- 

 stances into the higher symmetrical forms, even when it seems to 

 take place instantaneously, is never accompanied by an appreciable 

 heat-effect, nor by a measurable change of specific volume. 



A study of these phenomena from these points of view has been 

 made in several very convincing cases. Thus the temperature at 

 which the monoclinic, pseudotrigonal uranyl-magnesium-sodium- 

 acetate 1 ): NaMg(U0 2 ) 3 (C 2 H 3 O z ) + <?H Z is changed into an 

 apparently-real trigonal crystal, was determined to be 28 C. by 

 Steinmetz, who stated at the same time that the change observed 

 is accompanied neither by an appreciable dilatometrical, nor by a 

 thermal effect. 



Analogous results were obtained by Steinmetz with isopropyl- 

 amine-platini-chloride^}: (iso- C s H 7 .NH 2 ) 2 PtCl 6 , which is a mono- 

 clinic, but pseudo-rhombic substance, and which at 32 C. is changed 

 into an apparently-true rhombic individual, without measurable 

 heat-, or volume-effects. Something of the same kind was found 

 by Gossner 3 ) in the case of glaserite: K 3 Na(S0 4 ) 2 , and of the 

 corresponding chromate: K. d Na(CrO^) 3 ; here too, neither heat- nor 

 volume-effects were found when the monoclinic, pseudo-hexagonal 

 crystals passed into such of apparently-true hexagonal symmetry. 



Beautiful examples were also found by Gossner 4 ) in the case 

 of the tri-alkali-hydrosulphates'. K 3 H(SO /i ) 2 , (NH 4 ) 3 H(SO^) 2 , and 

 T/ 3 /^(S0 4 )o, and in that of the corresponding selenate: K 3 H(Se0 4 ) z . 

 The ammonium-, and potassium-hydrosulphates are monoclinic and 

 pseudo-trigonal; the thallo-sa.lt is really ditrigonal, with approxi- 

 mately the same angular values. On heating the monoclinic salts, a 

 system of three sets of lamellae, crossing at an angle of 60, becomes 

 visible, which at increasing temperatures get gradually more nume- 

 rous, until finally an apparently-perfect ditrigonal crystal is pro- 

 duced. The transformation is completely reversible, and according 

 to Gossner a continuous one. Fischer 5 ) however demonstrated 



1) G. Wyrouboff, Bull, de la Soc. Miner. 24. 93. (1901); Zeits. f. Kryst. 37. 

 192. (1903). 



2) A. Ries, Zeits. f. Kryst. 36. 329, 360. (1902); H. Steinmetz, Zeits. f. phys. 

 Chemie, 52. 449. (1905). 



3) B. Gossner, Zeits. f. Kryst. 39. 155. (1904). 



) B. Gossner, Zeits. f. Kryst. 38. 110, 159, 161. (1904). 



5) P. Fischer, Dissertation Gottingen, (1911), p. 10, 25. 



